Leishmania are obligate intracellular protozoan parasites of humans. Within infected patients, various species of this organism inhabit and destroy macrophages within the skin or internal organs (liver, spleen and bone marrow). Thus, they cause either ulcerative, nonhealing, disfiguring cutaneous lesions or degenerative and most often fatal visceral disease. These diseases afflict over 12 million patients in the tropics and neo-tropics worldwide. Our studies are aimed at understanding the mechanisms involved in the pathophysiology of this organism. In that regard, the basic cell, molecular and developmental biology of Leishmania and related trypanosomatid protozoa are investigated as models of human parasitism. How these parasites are able to survive, access nutrients, multiply and differentiate within their insect vector and mammalian hosts are questions central to understanding the basic parasitic nature and evolutionary adaptations of these organisms. Since these parasites interact directly with their hosts, knowledge of the composition and functions of their surface membranes seems essential. To that end, unique parasite surface membrane enzymes and other physiologically relevant proteins are identified and biochemically characterized to determine their functional roles in the survival of these organisms. Further, the genes encoding such proteins are being identified and characterized for the first time, toward defining their expression and regulation during course of parasite growth, differentiation and development. Further, experiments involving the targeted-deletion of such genes and the production of functionally null mutant parasites are in progress. The viability of such mutants will be tested in situ. Results of the latter studies should demonstrate whether the proteins encoded by these genes are, in fact, essential for the survival of these pathogens within their insect-vector and/or mammalian hosts. Thus, the current studies are aimed at defining the intrinsic needs of these organisms and determining their critical nature. In addition, these studies are of practical relevance toward demonstrating whether such parasite enzymes are logical targets for 1) the identification/design of new chemotherapeutic drugs, 2) the development of new diagnostic tools and/or 3) potential vaccines against these human pathogens.